SARA-G3 and SARA-U2 series - System Integration Manual
UBX-13000995 - R08
Objective Specification
Design-in
Page 135 of 188
2.7.2
Digital audio interface
SARA-G300 and SARA-G310 modules do not provide digital audio interface.
2.7.2.1
Guidelines for digital audio circuit design
The I
2
S digital audio interface of SARA-G3 and SARA-U2 series modules can be connected to an external digital
audio device for voice applications. The external digital audio device must be compliant to the configuration of
the digital audio interface of the cellular module, providing:
The opposite role: slave for SARA-G3 modules that act as master only; slave or master for SARA-U2 modules
that may act as master or slave
The same mode: PCM / short alignment mode or Normal I
2
S mode / long alignment mode
The same sample rate and serial clock frequency
Compatible voltage levels (1.80 V typ.), otherwise the lines must be connected by means of a proper
unidirectional voltage translator (e.g. Texas Instruments SN74AVC4T774 or SN74AVC2T245)
Figure 78 and Table 44 describe an application circuit for the I
2
S digital audio interface of SARA-U2 modules
providing voice capability using an external audio voice codec. DAC and ADC integrated in the external audio
codec respectively converts an incoming digital data stream to analog audio output through a mono amplifier
and converts the microphone input signal to the digital bit stream over the digital audio interface.
The module’s I
2
S interface (I
2
S master) is connected to the related pins of the external audio codec (I
2
S slave).
The
V_INT
output supplies the external audio codec, defining proper digital interfaces voltage level.
The external audio codec is controlled by the SARA-U2 module using the DDC (I
2
C) interface: this interface
can be concurrently used to communicate with u-blox GNSS receivers and with an external audio codec.
The
CODEC_CLK
pin of the SARA-U2 module (that provides a suitable digital output clock) is connected to
the clock input of the external audio codec to provide clock reference.
Additional components are provided for EMC and ESD immunity conformity: a 10 nF bypass capacitor and a
series chip ferrite bead noise/EMI suppression filter provided on each microphone line input and speaker line
output of the external codec as described in Figure 78 and Table 44. The necessity of these or other
additional parts for EMC improvement may depend on the specific application board design.
R2
R1
GND
U1
SARA-U2 series
Audio
Codec
26
SDA
27
SCL
SDA
SCL
19
CODEC_CLK
MCLK
GND
R3
C3
C2
C1
4
V_INT
VDD
1V8
MICBIAS
C4
R4
C5
C6
EMI1
MICLN
MICLP
Microphone
Connector
EMI2
MIC
C12 C11
J1
MICGND
R5
C8
C7
SPK
Speaker
Connector
OUTP
OUTN
J2
C10 C9
C14 C13
EMI3
EMI4
IRQn
BCLK
LRCLK
SDIN
SDOUT
36
I2S_CLK
34
I2S_WA
35
I2S_TXD
37
I2S_RXD
1V8
1V8
D3
D1
D4
D2
Figure 78: Application circuit for connecting SARA-U2 modules with an external audio codec to provide voice capability
